Bạn đã xem lại font chưa? Run Internet Explorer, click on Tools -> Internet Options -> choose page "General" -> Click on "Fonts" button -> Check:
Web page font: should be Times New Roman
Plain text font: should be Courier New
If you already have the correct font names, then perhaps you have installed an older version of these fonts, which do not have all Vietnamese characters. What version of MS Office did you install? MS Office 2000 and later should have newer versions of these fonts.
You can download newer version of MS fonts from this website:
Please let us know if this works for you.